Multilayer coating method

ABSTRACT

THIS INVENTION RELATES TO A MULTILAYER COATING METHOD WHICH IS CHARACTERIZED IN MAKING THE VISCOSITY OF A SECOND LAYER GREATER THAN THAT OF A FIRST LAYER WHEN TWO OR MORE COATINGS COMPOSITIONS PASS THROUGH A DOCTOR CHANNEL,   THE COATING COMPOSITIONS MAINTAINING THE RELATION OF SUPERIMPOSED LAYERS.

April 1971 MAMORU ISHIWATAAET AL 3,573,965

MULTILAYER COATING METHOD Filed Dec. 5, 1967 2 Sheets-Shet 1 FIG. I

INVENTORS MAMORU ISHIWATA YOSUKE UCHIDA YOSHIAKI NAGAI April 6, 1971MAMQRU s w T ETAL 3,573,965

MULTILAYER COATING METHOD Filed Dec. 5, 1967 2 Sheets-Sheet 2 (mm) 0-10Q 6 INVENTORS MAMORU ISHIWATA YOSUKE UCHIDA o YOSHIAKI NASA! u BY 6M 2.a, q I

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ATTORNEYS United States Patent O US. Cl. 117-83 1 Claim ABSTRACT OF THEDISCLOSURE This invention relates to a multilayer coating method whichis characterized in making the viscosity of a second layer greater thanthat of a first layer when two or more coating compositions pass througha doctor channel, the coating compositions maintaining the relation ofsuperimposed layers.

BACKGROUND OF THE INVENTION (1) Field of the invention This inventionrelates to a multilayer coating method for applying successively two ormore liquid coating compositions in layers to a continuously travellingweb-like support without conducting operations of setting or drying thecoating layers between applications of the coating compositions.

More particularly, this invention relates to an improved so-calledmultiple doctor coating method. The multiple doctor coating method is aprocess for coating a plurality of layers on a web which comprises (1)within a short clearance from the surface of the continuously travellingweb to be coated, providing a coating means having a plurality ofseparately closed chambers within said coating device, one for eachfluid coating composition, and a plurality of discharge openings,separated from each other by partitions and each connected to a chambersuch that said openings are positioned slightly apart from the surfaceof said web, (2) successively applying said coating compositions fromthe openings of the coating means without forming spaces between thecoating composition discharged from said openings while filling thespace between the surface of the web and the coating means with thecontinuously discharged coating compositions to form superimposed,multiple layers of the fluid coating composition between them, (3)withdrawing said filuid coating compositions from the coating means bythe travelling support, while maintaining the superimposed and distinctlayers thereof, and (4) then forming on the support the multiple layersof the coating compositions.

In particular, the method of the present invention is suitable forapplying at a high speed, in one step, two or more fluid coatingcompositions, such as photographic, light-sensitive, gelatino silverhalide emulsions, to a trave ling support, such as, a film base or abaryta paper in thin layers.

In the method described above, the thickness of each layer to be coated,that is, the volume of coating composition which passes through theclearance hereinafter referred to as a doctor channel between each ofthe doctor edges and the support, is determined by the clearance betweenthe doctor edge and the support, pressure difference between both endsof the doctor channel, etc., and can be wrtten as the followingequation,

3,573,965 Patented Apr. 6, 1971 ICC where i 2ti=Sun1 of coatingthickness from the first layer to its layer bi=Clearance between doctoredge and support corresponding to slot for coating composition of itslayer.

APi Pressure difference between the both ends of doctor channelcorresponding to slot for coating composition of its layer.

ki Proportional constant.

(2) Description of the prior art In hitherto known methods, in order tomake the coating thickness of each layer, especially the lowest layer,thinner, in one of the proposed methods the clearance between the doctoredges and the support have been made smaller. In another method, aso-called coating-composition pressure decreasing method, the coatingcomposi tion on the doctor channel has a pressure gradient whichincreases in the direction of travel of the support.

Such known methods however, have the following difliculties. In themethod which makes the clearance between the doctor edge and the supportsmaller, there is a tendency to produce comet-like spots due to othermaterials attached on the opposite surface of the support or on thesurface of the coating roller. Additionally, the coating thickness didnot become uniform across the whole width of the support, as the ratioof errors in the accuracy produced in the manufacturing of the doctoredges and the coating roller to the clearance between the doctor edgesand the support was increased. Moreover, when a jointed part of thesupport member is passing, it was necessary to move the coating head andcoating roller.

In the coating composition pressure decreasing method, as it is liableto admit air into the doctor channel from the environment, it wasnecessary to provide some kind of special device in the structure at theback and the sides of the doctor channel.

We, the inventors, found a coating method as described hereinafter tosolve such difiiculties in hitherto known methods.

Therefore, an object of the invention is to provide a coating method forforming these layers successively.

SUMMARY OF THE INVENTION This invention relates to a multilayer coatingmethod Which is characterized in making the viscosity of a second layergreater than that of a first layer when two or more coating compositionspass through a doctor channel, the coating compositions maintaining therelation of superimposed layers.

BRIEF DESCRIPTION OF THE DRAWING The invention will now be furtherdescribed with reference to the accompanying drawings in which:

FIGS. 1 and 2 are vertical cross-sectional views showing embodiments ofthe invention;

FIG. 3 is an enlarged vertical cross-sectional view showing the top partof the coating device shown in FIG. 1;

FIG. 4 is a model view of the distribution of fluid velocity of coatingcompositions at a doctor channel;

FIG. .5 illustrates the pressures of coating compositions at the outletof a slot and at the front and back of a coating device; and

FIG. 6 illustrates the relation between coating thickness and the ratioof viscosities of coating compositions.

3 DETAILED DESCRIPTION OF THE INVENTION This invention will be describedin detail with respect to the apparatus shown in FIG. 1 and FIG. 3 inthe case of a two layer coating by employing a calculated example.

In FIG. 1 and FIG. 3, the reference numeral 1 designates a continuouslytravelling web-shaped support which is to be coated. The numeral 2designates a roller which rotates with the support 1 by backing it up inorder to keep it running constantly. The numeral 3 is a coating headwhich is located on the side of support 1, the coat ing devicemaintaining a slight clearance with it. The coating head 3 comprisesliquid chambers 4 and 5 which supply coating compositions for the firstlayer (the lower layer) and the second layer (the upper layer), slots 8and 9 for each of the coating compositions which are respectivelyconnected to the liquid chamber 4 and 5 and are opened toward thesupport member 1 across the whole width thereof. Doctor edges 12 and 13form spaced walls to isolate the slots from each other and have smoothedsurfaces refined in high accuracy opposed to the support and form doctorchannels 16 and 17 between the smoothed surfaces and the support. Theliquid chambers 4 and 5 are respectively connected to liquid feedingequipment which supplies the coating compositions to complement only thequantity derived by the support. The first and the second coatingcompositions pass through the doctor channel 17, the coatingcompositions being superimposed one upon the other. The distribution offluid velocity is shown in FIG. 4. In FIG. 4, A is the case in which theviscosity of the first coating composition is larger than that of thesecond coating composition, B is the case in which both are equal, and Cis the case in which the viscosity of the second coating composition islarger. The quantity of coating composition passed through the doctorchannel can be obtained as an integral value of fluid velocity. Asunderstood from FIG. 4, the quantity of coating composition passedthrough the doctor channel in the case C is smaller than quantity ofcoating composition in the case A or B. Moreover, in the case where theviscosity of the second layer coating composition is larger than that ofthe first layer, resistance exerted when the coating composition passesthrough the doctor channel becomes larger, so that the static pressureof the second layer coating composition at the outlet of slot 9 becomeshigher and can give a pressure gradient to the coating composition atthe doctor channel 16, the pressure gradient being increased in thetravelling direction of the support, or can make the pressure gradientlarger, whereby the quantity of coating composition for the first layerbecomes smaller.

As described above, by making the viscosity of the coating compositionfor the upper layer larger than that of the lower layer, the quantity ofcoating composition passing through the doctor channels 16 and 17becomes smaller, so that coating of a thinner layer is possible. Inorder to be more clearly understood, a calculated example is shown inthe case where the coating is performed under the following conditions.

COATING CONDITIONS Length of doctor edge 12 along travelling directionof support: 11 :3 mm.

Length of doctor edge 13 along travelling direction of support: (1 :4mm.

Clearance between support and doctor edges: b=0=.3 mm.

Pressures in the front and back of coating device:

p =p =O mm. gauge Velocity of support: V=20 m./ min.

Quantity of coating composition for lower layer t '-=quantity of coatingcomposition for upper layer t Viscosity of coating composition for lowerlayer: n =20 Viscosity of coating composition for upper layer: [cp.]

4 Static pressure of coating composition for lower layer at exit ofoutflow slot: P [mm. H O gauge] CALCULATED RESULT A l0 0. 5 0. 215 .B20 1. U 0. 191 C 5. 0 0.

1 is shown in FIG. 6.

As clearly understood from the above description and calculated example,when coating according to this invention is carried out, it is possibleto make the thickness of coating composition coated on the supportthinner without narrowing the clearance between the doctor edges and thesupport. Therefore, difficulties, such as generation of comet-likespots, accurate uniformity of coating thickness, difliculty in passingthrough jointed parts, etc., produced due to narrow the clearancebetween the doctor edges and the support, would be dissolved.Additionally, in this method it is not necessary to provide any kind ofspecial device in order to protect against the introduction of air.

When three or more coating compositions are coated according to thisinvention, for example, when four layers are simultaneously coated, itmay be achieved by employing an apparatus such as the one shown in FIG.2, under conditions that the viscosity of the coating composition forthe second layer is larger than that of the first layer (liquidreservoirs 6 and 7, slots 10 and 11, doctor edges 14 and 15, etc., arethe same as in FIGS. 1 and 3). In this case, it is also desired that theviscosity of the coating compositions for the third and fourth layers belarger than that of the first and second layers, but, even if the formeris smaller than the latter, the process would be effective to someextent.

AN EMBODIMENT OF THE INVENTION By employing the apparatus shown in FIG.1, a photographic emulsion and gelatinous aqueous composition werecoated on subbing cellulose triacetate film under the followingconditions:

Length of doctor edge 12 along travelling direction of support: a =3 mm.

Length of doctor edge 13 along travelling direction of support: 11 :4mm.

Clearance between edges and support: b=0.3 mm.

Pressures in the front and back of coating device: P

P =atmospheric pressure.

Velocity of support: V=20 m./min.

Viscosity of coating composition (photographic emulsion) for lowerlayer: ,u =2O cp.

Viscosity of coating composition (gelatinous aqueous solution) for upperlayer: 100 cp.

Quantity of coating composition for lower layer=Quantity of coatingcomposition for upper layer.

When the viscosity of the coating composition for the upper and lowerlayers is identical, 20 cp. respectively, the coating thickness was0.165 mm. By comparison, with the larger viscosity in the second orupper layer as shown above, the coating thickness under the aboveconditions became 0.115 mm., that is, about 30% reduced. As a result ofchecking photographic film obtained via setting and drying aftercoating, there were no comet-like spots,

no mixing between layers, and uniform thickness. Therefore, quality wasexcellent.

While we have shown and described certain specific embodiments of ourinvention, we are aware that many modifications thereof are possible.Our invention, therefore, is not to be limited by dimensions and contourof the apparatus shown, by the kind of coating compositions, etc.

What is claimed is:

1. In a process for coating multiple, distinct layers on a web backed bya suitable surface and travelling at a high rate of speed comprisinglocating adjacent to said web a coating head having only a plurality ofdischarge openings directed toward said web, a plurality of chamberseach connected to a respective discharge opening with a doctor bladeforming a downstream portion of each discharge opening in the directionof travel of said web and located in close proximity to said web,supplying fluid coating compositions into each of said chambers,applying said coating compositions successively to the surface of saidweb from said discharge openings without forming spaces between saidcoatings while filling the space between the surface of the web and saiddoctor blades with said coating compositions, maintaining said coatingcompositions between said web and said doctor blades under pressure andmetering said coating compositions by means of said doctor blades toform superimposed and distinct multiple layers of said fluid coatingcompositions on said web, the improvement comprising the steps of:providing coating compositions for said chambers having viscositieswhich increase in order from the lowest layer formed thereby to theuppermost layer, and varying the viscosity of respective compositionswhile maintaining said order to thereby control the thickness of thelayers of said multiple layer coating.

References Cited UNITED STATES PATENTS 2,761,791 9/1956 Russell 117-832,932,855 4/1960 Bartlett 117-72 3,206,323 9/1965 Miller et a1 117-34WILLIAM D. MARTIN, Primary Examiner W. R. TRENOR, Assistant Examiner US.Cl. X.R.

